1. Field of the Invention
The invention relates to a steel suitably utilized in a rolling part such as a raceway ring and a rolling body of a roll bearing used in automobiles and various machines. More particularly, it is directed to an improvement in the strength of the steel when the steel is cold worked.
2. Description of the Related Art
A rolling part of a roll bearing is usually formed by following the steps of plastic-working, turning, heat-treating, grinding, and surface-finishing a bearing steel. If the steel is, e.g., a high carbon chromium bearing steel (hereinafter referred to as "bearing steel") which is often used as a general-purpose bearing steel, the heat treatments used for it are quenching and tempering. Further, if the steel is, e.g., a case hardened bearing steel used as a carburized bearing steel (hereinafter referred to as "carburized steel"), then the heat treatments used for it are carburizing, quenching, and tempering.
On the other hand, a rolling part, such as a raceway ring, is fabricated by cold working the bearing steel or the carburized steel to skip the turning process, and heat treating it thereafter.
By the way, since a rolling part of a roll bearing is subjected to repeated stress under a high surface pressure, flaking due to rolling contact fatigue comes to appear on the raceway surface or on the rolling surface in due course. As causes of a reduction in the total number of rotations that induces damage on the material by the rolling contact fatigue, i.e., a reduction in the life against the rolling contact fatigue (hereinafter referred to as "rolling life" or simply as "life") the following are known.
(1) Flaking is caused by microcracks that are disseminated with a damage as a starting point, the damage being caused on the surface of a bearing by foreign matter such as chips, shavings, burs, abrasion powder improperly introduced into a bearing lubricant. PA1 (2) Nonmetallic inclusions are present in the base material of the bearing steel and they serve as a source for stress concentration due to their hardness being high and their plastic deformability being low. As a result, they hamper the effect of alleviating the stress concentration to decrease the life against the rolling contact fatigue.
The applicant studied in various ways the extension of the life of the roll bearing steel and the control of incidence of cracks during its pre-working process such as forging involving a high working rate in the fabrication of a bearing, obtained some findings on the relationship between the amount of retained austenite and the life as well as the relationship between the sulfur content and the crack incidence during the pre-working process, etc., and disclosed an invention based on these findings in Japanese Patent Unexamined Publication No. 125841/1990. According to the disclosure, sulfur becomes the source for generating sulfide nonmetallic inclusions such as MnS, and the generated MnS acts as an origin for causing cracks during a bearing part pre-working process such as forging and rolling. To overcome this problem, medium carbon manganese steel whose sulfur content is 80 ppm or less was used and the amount of retained austenite was set to 25 to 45 vol. % on a surface portion of the bearing part after carburizing or carbonitriding heat treatment, and as a result, a roll bearing which can enjoy a long life even under a lubrication having foreign matters, which can prevent generation of cracks during its pre-working process such as forging, and which can improve its working rate could be provided.
Further, the applicant could successfully improve the life of the bearing even if its plastic working rate was made higher by using an alloy steel having a carbon content of 0.3 to 0.6 wt. %, a chromium content of 3 to 14 wt. %, and a sulfur content of 80 ppm or less, heat treating it after carburizing or carbonitriding, improving its surface hardness while controlling the amount of fine carbides in the surface portion to 20 to 50 vol. %,, and controlling the amount of retained austenite in the surface portion to 10 to 25 vol. % to thereby obviate incidence of microcracks under the lubrication having foreign matters (see U.S. Pat. No. 5,030,017).
As described above, it was certainly effective to control the S content in the steel to 80 ppm or less to prevent pre-working cracks for the rolling part, whose rolling life was improved. However, it was impossible to completely eliminate the MnS-attributed cracks by merely regulating the S content as in the conventional way. Even if no cracks are identified at all from observation by naked eyes or a stereoscopic microscope, reduction in the strength is found on the worked rolling part. Particularly, when a rolling part such as a raceway ring is cold-worked by skipping the turning process, reduction in the ring rotating fatigue life against fatigue-induced cracks is further promoted than a turned part.